For a power conversion device in recent years, innovation in technology of a power semiconductor module, which is a main component of the power conversion device, has realized faster switching operation to reduce loss generated by this power semiconductor. Thus, particularly, a cooling unit can be downsized, and as a result, a power conversion device can be downsized. Particularly, an uninterruptible power-supply system called UPS is often installed in the surrounding of a large city, where the land prices are high, for a data center, and it is accordingly desired that the installation area is small.
An uninterruptible power-supply system is configured including, in addition to semiconductor modules, busbars, by a fuse for protection against short-circuit fault, capacitors for smoothing a direct current power, and the like. Downsizing of an uninterruptible power-supply system also essentially requires downsizing of capacitors in addition to downsizing of a power semiconductor module.
For downsizing of capacitors, reduction in the number of capacitors in parallel by reduction in the capacitance is essential. In order to reduce the required capacitance of a capacitor, reduction in the ripple current of a converter is essential.
Technology related to reducing ripple current of a power converter is described, for example, in Patent Document 1. In PROBLEM TO BE SOLVED of Abstract of Patent Document 1, described is “To provide a power converter for suppressing ripple current flowing in a current smoothing capacitor.” In SOLUTION of Abstract, described is “Three-phase bridge circuits 1A, 1B are connected to a power source comprising a battery Vb and the current smoothing capacitor C in parallel. Each bridge circuit operates in accordance with a PWM signal from drive circuits 11A, 11B, and power-converts to drive a first motor 25A and a second motor 25B. Triangular wave generators 7, 8 generate triangular waves TR1, TR2 having a phase difference of 90 degrees. When both the motors are during power running or regenerative operation, the PWM signal driving the first motor is generated on the basis of the triangular wave TR1, and the PWM signal driving the second motor is generated on the basis of the triangular wave TR2. The phase of the ripple current generated in each bridge circuit by the phase difference between both the triangular wave signals is shifted to 180 degrees to be canceled so as to suppress the ripple current flowing in the current smoothing capacitor C small.”